1. Field of the Invention
The present invention relates to an apparatus and method for polishing a workpiece, and more particularly to an apparatus and method for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused on by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. An abrasive cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the abrasive cloth and clamped between the top ring and the turntable. An abrasive solution containing abrasive grains is supplied onto the abrasive cloth and retained on the abrasive cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the abrasive cloth is therefore polished to a flat mirror finish while the top ring and the turntable are rotating.
After polishing, the used abrasive solution containing abrasive grains, ground-off particles of the semiconductor wafer and the like remain on the abrasive cloth mounted on the turntable. Further, after polishing, a surface condition of the abrasive cloth is deteriorated by the polishing operation. As a result, a polishing rate is not constant throughout the whole polishing time, and the change in the polishing rate occurs on the polishing surface of the abrasive cloth, resulting in a failure to accomplish a desired degree of flatness of the polished surface of the semiconductor wafer and shortening a service life of the abrasive cloth.
A change in polishing characteristics of the abrasive cloth with time depends on material for the abrasive cloth, a surface configuration of the abrasive cloth, the type of abrasive solution to be used, or a combination thereof. For example, in case of abrasive cloth comprising nonwoven fabric composed of fibers bound together by urethane resin, the property of the surface of the cloth changes with the polishing operation. In case of abrasive cloth comprising polyurethane foam, micropores in its surface are clogged with abrasive grains, and dulling or glazing occurs in micropores in its surface. As a result, the retaining condition of the abrasive solution or the abrasive grains changes, and hence the polishing characteristics of the abrasive cloth changes. Further, the polishing action is affected by various factors including grain sizes of abrasive grains, the degree of intensity of abrasive solution in chemical etching action and the like.
In order to eliminate the above difficulties or disadvantages, after a polishing operation finishes once, a certain treatment is applied to the abrasive cloth to regenerate the abrasive cloth. This treatment is called a dressing of the abrasive cloth. The object of the dressing is to remove the abrasive solution containing abrasive grains and ground-off particles of the semiconductor wafer and to restore a surface condition of the abrasive cloth. The dressing is carried out after polishing a preceding semiconductor wafer and before polishing a subsequent semiconductor wafer.
Conventionally, the dressing is carried out to remove the abrasive solution and ground-off particles of the semiconductor wafer and to regenerate the abrasive cloth by scrubbing the abrasive cloth with a brush while supplying a dressing liquid such as water onto the abrasive cloth. Alternatively, the dressing is carried out by pressing a dressing tool having diamond grains on its lower surface against the abrasive cloth while supplying a dressing liquid such as water. The above dressing methods can be used properly depending on the types of abrasive cloth.
However, the used abrasive solution and ground-off particles of the semiconductor wafer are being accumulated while a polishing operation is carried out. Further, the abrasive cloth is being deteriorated throughout the whole polishing time. Therefore, the polishing action by the abrasive cloth is being weakened in the course of a polishing operation.
Further, since the dressing must be performed in between polishing operations, the polishing operation cannot be continuously performed, thus lowering the throughput of the semiconductor wafers.